1. Field of the Invention
The present invention relates to a method of discriminating a through-wall crack and measuring the through-wall length of the crack formed in a steam generator tube of a nuclear power plant.
2. Description of the Prior Art
In general, steam generator tubes of a nuclear power plant often undergo various kinds of corrosion and mechanical damage under the operation condition, such as stress corrosion cracking, pitting, inter-granular attack, wear, etc. Such corrosion and degradation are principal factors that affect the safety and integrity of major components of nuclear power plants.
In particular, the occurrence of through-wall cracking of a steam generator tube may result in the serious contamination of a system and environment due to the leakage of primary coolant, including the radioactive contamination of a secondary side. Therefore, it is very important to develop an advanced technique to discriminate the evolution of through-wall cracking in an early stage and to measure its size and length accurately.
The formation and growth of all kinds of defects in a steam generator tube are usually inspected and evaluated by a nondestructive test during a periodic in-service inspection that is normally carried out after each operation cycle.
For a steam generator tube inspection, an eddy current test is usually employed in order to detect and measure various kinds of defects. Generally, a bobbin coil probe and a motorized rotating probe are mainly used.
For volumetric defects formed in a steam generator tube, their sizes can be basically assessed in terms of percentage of defect depth to tube wall thickness taking into account of phase angle in eddy current signal characteristic related to these kinds of defects. However, this evaluation has a very low confidence level.
For crack defects, on the other hand, the detection of cracks themselves and the quantitative evaluation of crack sizes using eddy current test are difficult in comparison with volumetric defects.
Since these technical difficulties, a testing method for detecting a crack, a quantitative evaluation method for determining crack size, and a proper measure criterion for a defected tube are not clearly prescribed even in eddy current testing standards such as the American Society of Mechanical Engineers (ASME) codes.
Furthermore, it is almost impossible to determine whether a crack completely penetrates the wall thickness of a steam generator tube and to measure the through-wall length of the crack.
Currently, in nuclear power plants, the presence of a through-wall crack of a steam generator tube is determined by an in-situ pressure test that is an indirect way to assess a cracking condition of a suspected tube by observation on a leakage of coolant through a crack or a pressure change of coolant.
This mechanical test method is disadvantageous in that a widening and permanent deformation of the crack may arise from an intentional pressurization to a primary or secondary side, the measurement of a through-wall crack length is not possible, the test results are not reliable, an additional testing time and costs are consumed and large-sized equipments are required.
Therefore, it is required to develop a method that can discriminate the through-wall crack and measure accurately through-wall length of the crack using the eddy current signals collected from periodic inspections.
The present invention relates to an eddy current test method which can discriminate the through-wall crack formed in a steam generator tube and measure accurately its through-wall length using a motorized rotating probe.
In more detail, the present invention is characterized in that a through-wall crack is discriminated and the through-wall length of the crack is measured by relating the characteristic amplitude values of eddy current signals, which are generated whenever a pancake coil, or a combination of a pancake coil and one of a plus point coil, an axial coil and a circumferential coil circumferentially provided on the surface of a motorized rotating probe passes the crack while being spirally rotated, to the positions in the length of the crack.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide an unique nondestructive method, which is capable of precisely determining whether a crack formed in a steam generator tube of nuclear power plant is a through-wall or non-through-wall crack and precisely measuring the through-wall length of the crack using an amplitude curve of the eddy current signals of a motorized rotating probe.
In order to accomplish the above object, the present invention provides a method of discriminating the through-wall crack and measuring the through-wall length of the crack in the steam generator tube of a nuclear power plant, characterized in that the through-wall crack is discriminated and the through-wall length of the crack is measured in such a way that the number and positions of inflection points are measured on a graph that is plotted by relating the characteristic amplitude values of eddy current signals to the corresponding positions in the length of the crack, which are generated whenever a coil of a motorized rotating probe passes the crack while rotating spirally, moving along the length of the crack.
Preferably, the coil of the motorized rotating probe should be composed of a pancake coil, or a combination of a pancake coil and one of a plus point coil, an axial coil and a circumferential coil.
The method may be carried out using a computer program.